Small signal impedance modelling and stability assessment of HVDC systems

Student thesis: Doctoral Thesis

Abstract

With high penetration of converter interfaced renewable energy and distributed generation, and increased use of HVDC interconnections, the characteristics of power systems is undergoing significantly changes. Interaction between grid-connected converters and networks is likely to increase, which may lead to stability and resonance problems, and in particular, when the grid is “weak” as the relatively high system impedance. Therefore, that is important adequate method is developed for assessing system stability. This thesis presents the small signal impedance modelling of grid-connected 2-level voltage source converters (VSC) and modular multilevel converter (MMC) for system stability assessment.In the case of 2-level VSC connected to weak grid system, the VSC impedance is mapped into the positive-negative (pn) sequence-frame for ease of analysis, and the stability problem associated with the coupling admittance is studied. It is found that traditional outer-loop controllers (e.g., active/reactive power and AC voltage controllers) create high coupling admittance that has negative impact on system stability. Improved outer-loop controllers are proposed and to improve the system stability which add compensation terms into the d and q-axis at the potential resonance frequency range. Small signal analysis and time domain simulation confirm the effectiveness of the proposed method. Large number of MMC based HVDC systems for interconnection or offshore wind farm integration are already in operation and many more will be installed in the coming years.MMC has multiple internal harmonics, which causes complex internal dynamics and multifrequency response. To accurately model the multiple frequency response and include all internal harmonics dynamics with MMC, the harmonic state-space (HSS) modeling approach is adopted. A detailed procedure for deriving the small-signal model of single-phase MMC system using HSS modelling approach is presented first. To address the issues related to single-phase MMC modelling including the existence of zero-sequence current and the use of controllers in abc frame, which are not in accordance with practical 3-phase system, impedance modelling and validation of the three-phase MMC based on HSS are conducted. In order to simplify the analysis on the coupling characteristics between different frequencies in MMCs, the proposed model is developed in pn frame, where the zero-sequence current in three-phase three wire system is modelled in a simple way.A simplified 2 by 2 admittance matrix in pn frame is extracted from the MMC small-signal model for ease of system stability analysis. Different outer-loop controllers, operating points and working mode are adopted and compared in the analysis to illustrate the effects system stability. It is found that for AC grid with single MMC, high PLL bandwidth leads to a less stable system. Compared with inverter mode, MMC in rectifier mode is more likely to induce system instability. Using the developed impedance model, the multi-infeed interaction factor (MIIF) measure is adopted to analyze the interactions for multi-infeed converter systems. Detailed studied are carried out for an AC network with two MMCs considering different MIIIF. Analytical studies and time-domain simulation results show that system with high MIIF where strong couplings between the two MMCs exist may lead to instability.
Date of Award9 Oct 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorLie Xu (Supervisor) & Hong Yue (Supervisor)

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